Estuary Study: Non-Native Species Drive Oyster Decline

Jul 27, 2009

Led by marine biologist David L. Kimbro, now of The Florida State University (FSU) Coastal and Marine Laboratory, the study shows that in Tomales Bay, half the population of California’s native Olympia oyster has perished because its habitat has fallen victim to the dysfunctional relationship between native and non-native species.

The work of Kimbro, a postdoctoral associate, and colleagues from the University of California-Davis and its Bodega Marine Laboratory has magnified concerns that predator-prey mismatches between native and exotic species may lead to further losses of critical habitats that support marine biodiversity and ecosystems.

“What David Kimbro’s research reveals about California’s Olympia oyster raises a specter for Florida as well,” said Felicia Coleman, director of the FSU laboratory.

“First, our state has a remarkably productive native oyster population at risk in the Apalachicola Bay,” she said. “These oysters are sustainably harvested and provide critically important settlement habitat for an amazing array of species, so its loss would be devastating. Second, we know that in Florida’s benign climate, exotic species are pretty easily established. One need only follow the trajectories of lionfish, Australian pine, Brazilian pepper and Burmese python for a grim view through the looking glass.”

Kimbro and colleagues describe their findings in a paper (“Invasive species cause large-scale loss of native California oyster habitat by disrupting trophic cascades”) that was recently published in the online and print editions of the peer-reviewed journal Oecologia.

Their study is expected to help guide habitat management and conservation efforts, including plans now being drawn up along the West Coast to restore native oysters.

In California, said Kimbro, the artificial predator-prey mismatch has allowed high numbers of invasive snails to destroy the biological diversity of Tomales Bay, a coastal estuary about 40 miles northwest of San Francisco, by consuming and eliminating oyster habitat.

As an example, he points to the invasive European green crab from the Atlantic Coast (inadvertently introduced from Europe, so an exotic species even there). Smaller than California’s native crab, it is less effective at controlling the snail population because it uses only brute force to kill its prey -- unlike its native counterpart, which is capable both of crushing the snails and using its claws like a can opener to peel open hard-to-crush larger shells.

“You’d think similar-looking invasive and native species could be swapped in a system without adverse effects,” he said. “But we have shown that, functionally, this is not the case.”

In Tomales Bay, the exotic species occupy its inner reaches, where saline levels are lower. However, native predators cannot tolerate the inner bay’s lower salinity, so they can’t help to check the invasive snail population proliferating there. The invasive crabs are stuck in the inner bay because the natives hold the middle, with its higher salinity.